Water-assisted injection molding is moving past its status as a tech-paper darling, as equipment companies rolled out commercial versions during trade shows last year.
The technology, a cousin to gas-assisted molding, injects a shot of pressurized water into the mold to pack out the melted plastic, then cool it off quickly. Suppliers say water can find a niche in large hollow parts, and especially in long, rod-shaped parts with thick walls or long flow paths.
``This is difficult to do with gas-assisted molding,'' said Helmut Eckardt, technical director of Battenfeld GmbH of Meinerzhagen, Germany.
The automotive coolant duct became the poster child for water molding at K 2001 last October. At least three booths demonstrated the water-assist process by molding ducts during the show in Dusseldorf, Germany: Germany's IKV technical school on a Battenfeld injection press, Engel Vertriebsgesellschaft mbH and Cinpres Gas Injection Ltd. The complex ducts come with molded-in bends, tabs and connection ports, features normally made by technical blow molding or by post-forming and assembly.
Another company, Alliance Gas Systems Inc., picked Plastics Encounter Atlanta in November to introduce its Hydrojection technology. Alliance offers versions that use water, and both nitrogen gas and water, to mold parts.
Although they share some basic principles, suppliers agree that water-assist molding will not replace gas-assist.
``They're two different animals, and they're done for different reasons,'' said Steven VanHoeck, vice president of Alliance Gas Systems.
The process is application- and mold-specific, its backers say.
``[Water] has its exclusive purposes, and No. 1, 98 percent of the time, will be cycle-time reduction,'' VanHoeck said.
Fast cooling is the strongest point for a simple reason: the thermal conductivity of water is 40 times higher than nitrogen gas, according to Battenfeld. Water literally washes the heat away and out of the part. On heavy-wall parts it adds up: Water-assist can reduce cooling times by 30-70 percent compared with gas, according to suppliers.
Molding with water has some obvious downsides. GE Plastics' Jack Avery, an expert in molding technologies including gas-assist, points out that a little nitrogen leaking from a mold is pretty harmless, but water can cause problems.
``I have a couple of questions about it. The first is, they have to make sure they get rid of all the water,'' said Avery, manager of operational assets.
Avery is not alone.
``The main problem? It's getting the water out of the part; this is still the big issue,'' said Hermann Plank, managing director of Ferromatik Milacron Europe/USA.
IKV, in a presentation at K'Plast 2001, held just before K, listed five basic ways to remove water from the finished part. Before the mold opens, you can blow it out with compressed air, drain it out using gravity, evaporate the water or coinject a liquid blowing agent. For complex shapes or curved parts, it may be necessary to remove water in a separate step, outside the mold.
The melt can be injected as a short shot, or the water can push excess melt to a secondary holding area.
Most systems filter and reuse the water.
The concept of using water during injection molding dates to the late 1930s, according to IKV, the Institut fur Kunststoffverarbeitung in Aachen, Germany. A few patent applications in the 1970s mentioned water, but nothing much happened until IKV picked up on it four years ago. Officials of Alliance Gas Systems say they began working independently on water-assist several years ago in the United States, with no knowledge of the IKV project.
IKV researchers hit upon plain old water after they tried unsuccessfully to find a gas that was better than nitrogen. Sometimes gas causes the material to foam; that doesn't happen with water. IKV said water achieves greater part dimensions and diameters with lower residual wall thicknesses. Water also produces smoother inner walls.
The IKV booth at K 2001 marked the first time Battenfeld's Aquamould process was operated at a trade show. Visitors crowded the technical school's tiny booth to watch yellow polypropylene ducts fall from a conveyor into a bin, coming out of a 230-ton Battenfeld press. Battenfeld noted that water eliminates problems that occur when sections of a part with thick walls butt up against thin-wall areas.
Engel of Schwertberg, Austria, demonstrated its Watermelt process at K by molding an automotive coolant tube on a 165-ton Victory press. The nylon part has an outside diameter of 25 millimeters and a wall thickness of 3-4 millimeters.
The cavity was partially filled with melted plastic. Then an injector near the gate released water to press the melt against the mold wall, displacing some of the melt into a secondary cavity. As the melt reached a second injector, that one kicked in with water, too.
Kai Jacobsen, Engel North America's manager for process engineering development, said Engel's design makes getting the water out no problem - although customers still ask about leaks.
``We're pretty much at a stage where the unit itself is sound,'' he said.
Last year, British gas-assist competitors Cinpres Ltd. and Gas Injection Ltd. merged. The resulting company, Cinpres Gas Injection Ltd. of Nantwich, England, demonstrated its water-assist process by molding a duct from glass-filled nylon during K 2001. The firm claimed cycle times half those of conventional gas assist.
The process, dubbed water injection molding, was developed by Factor Maschinen & Anlagentechnik GmbH, a Hainburg, Germany-based distributor of Cinpres Gas Injection products.
Ferromatik Milacron developed its Aquapress water-assist process in Malterdingen, Germany.
Aquapress was presented at the Molding 2002 conference, held March 4-6 in New Orleans. First, the cavity is completely filled with melted plastic. The swell pressure stage lasts until the skin hardens, then water is injected, pressing the excess core material back into the retraction chamber of the screw. Compressed air blows out the water.
Arburg GmbH + Co. of Lossburg, Germany, is working on water-assist but does not have a commercial product yet, a company spokesman said.
Alliance Gas Systems of Chesterfield Township, Mich., near Mount Clemens, markets three sequence variations, with each following the injection of resin:
* Water and gas. A part is configured vertically up and down, and gated at the bottom. After the water goes in, the injector switches over and injects a small bubble of nitrogen, which rises to the top of the cavity. When the manifold switches back to vent water out again, the bubble can expand and push the water out of the cavity.
* Water, followed by a shot of gas to evacuate the water.
* Gas injection, followed by water for cooling, then another shot of gas to expel the water. This process combines aspects of gas-assist and water-assist.
VanHoeck of Alliance stressed that product design, geometry and resin choice will determine which sequence is right for a given part. But he thinks the third version could end up with more than 50 percent of the entire water-assisted molding market. The water-and-gas combination works particularly well with molding polyolefins, he said.
Alliance makes stand-alone Hydrojection units, which also can be used with the company's LGC series of gas-assisted molding equipment, as well as units that integrate both processes. A new electronic injector pin, the HMP-3, boasts a high flow level, opens and closes quickly, and can alternate between water and gas.
Sample products include a polyethylene shovel handle now being made by gas-assist, car door handles and a tubular, wraparound handle for a chainsaw.
Like other water-advocates, VanHoeck said water-assisted molding makes the most sense for parts that take a long time to cool. How long?
``I usually say you can take a shot, go have lunch and come back and take out the part.''